1. Field of the Invention
The present invention relates to a copying machine for both black-and-white and color.
2. Description of Related Art
A generally known copying machine has a scanning optical system, a photosensitive means and a transfer means. The scanning operation is repeatedly performed by the scanning optical system so that a latent image of an original is formed by the photosensitive means and developed by a toner. Thereafter, the toner image is transferred onto a transfer paper held on the transfer means rotated in proximity to the photosensitive means to obtain a copy of the original.
In the above-mentioned copying machine, a transfer drum as one of the transfer means is opposed to a photosensitive drum as one of the photosensitive means and contacts this photosensitive drum. In the case of the color copy, a color original image is decomposed into a plurality of colors by a color filter by means of the scanning optical system for scanning the original above the photosensitive drum and the decomposed color image is successively exposed. The latent image of the exposed color image is developed by a relative complementary color toner every exposure, and each color toner image is repeatedly transferred onto the transfer paper held on the transfer drum to obtain a copy thereof. At this time, the transfer paper is rotated while this paper is grasped by a clamper of the transfer drum, and a required number of transferring operations(three times in the case of the full color) are performed.
The colors generally used in the development are composed of yellow(Y), magenta(M) and cyan(C). In the conventional copying machine for both black-and-white and color, a developer of black(BK) is further disposed in addition to developers of such three colors so as to provide a copy of black and white colors as well as the copy of the full color.
There is a difference between the full color copy and the copy of the black and white colors with respect to the construction as to whether the color filter is used in the exposure or not. Accordingly, in the case of the same exposure amount, the light amount of an image formed on the photosensitive body is different between the full color copy and the copy of the black and white colors.
Therefore, in the case of the copy of the black and white colors, the exposure is performed through a filter to reduce the light amount to be similar to the full color copy, and a voltage applied to an exposure lamp is increased in the case of the color copy such that the light amount on the photosensitive body is set to be equal to that in the case of the copy of the black and white colors. Such an operation has a lot of loss in control.
It is not necessary to control the exposure amount if process speeds such as respective drum circumferential speeds are changed in the cases of the full color copy and the copy of the black and white colors.
However, in the conventional color copying machine, the photosensitive drum and the transfer drum are connected to each other by gears to provide an integrally associated drive system. Accordingly, the copy processing can be executed only at a single process speed.
This is because it is difficult to switch more than two kinds of process speeds by the engagement of the gears, the slip of clutches, etc.
Japanese Laid-Open Patent Publication No. 62-187365 discloses a color copying machine for servo-controlling the photosensitive drum, the transfer drum and the scanning optical system by separate motors.
Such a color copying machine is provided with a scanning optical system, a first motor for driving the scanning optical system, a photosensitive means, a second motor for driving the photosensitive means, a transfer means, and a third motor for driving the transfer means.
In the above color copying machine of the Japanese publication, the motors are separately disposed with respect to the photosensitive means and the transfer means so that the connecting gears and the clutches as in the conventional copying machine are not disposed. Accordingly, in such a copying machine, it is possible to change the process speeds in principle.
However, the above technique is applied to a copying machine having only a full color copying function. Accordingly, such technique does not disclose a copying machine having functions of both the full color copy and the copy of the black and white colors, in which the process speeds are suitably switched corresponding to at least two kinds of required copy modes.